Hydraulic lash adjusters are well known for use in internal combustion engines. Lash adjusters are typically used to eliminate clearance or lash between engine valve train components that can result from varying operating conditions. Hydraulic lash adjusters are used to maintain engine efficiency, to reduce engine noise, and minimize wear on the valve train.
Hydraulic lash adjusters operate by transmitting the energy of a valve actuating cam through hydraulic fluid trapped in a pressure chamber beneath a plunger in the lash adjuster body. During each operation of the cam, as the length of the valve actuating components vary (due to temperature changes, for example), small quantities of hydraulic fluid are permitted to enter or escape from the pressure chamber. As the hydraulic fluid enters or escapes the pressure chamber (leak down), the position of the plunger is adjusted and consequently the effective total length of the valve train is adjusted which minimizes or eliminates the lash.
Conventional hydraulic lash adjusters have a leak down rate controlled by precise clearance between two concentric tubes, namely, the plunger and the outer cylinder, such as disclosed in U.S. Pat. No. 5,622,147. The leak down rate is controlled by a leak path located between the outer periphery of the plunger and the inner wall of the lash adjuster body. Since the leak down rate of these prior lash adjusters depends on the magnitude of the gap between the two concentric tubes raised to the third power, slight changes in dimensions can have a large effect on the leak down rate. As a result, these tubes typically are provided with a lapped or polished finish and are matched to provide a leak path of the appropriate dimensions to ensure the required accuracy in leak down rate. The process used to provide tubes with these precise dimensions in order to achieve the desired accuracy is an expensive process.
To properly minimize lash, the leak down rate must be sufficiently fast so that as the engine valve heats and expands, the lash adjuster can relax and accommodate the expansion. If the lash adjuster does not accommodate the engine valve expansion, the engine valve may not seat completely. The inability of a lash adjuster to accommodate engine valve expansion could potentially cause engine problems such as loss of power output and deposit buildup on the engine valve stem. These problems can be exacerbated with new engine designs that heat the catalyst more rapidly causing the engine exhaust valves to also quickly heat and expand.
While hydraulic lash adjusters typically can increase their length quickly, they require more time to shrink, which is a function of the oil viscosity and temperature. For example, as the engine's oil gets cooler and more viscous, the leak down rate decreases. However, the engine valve train growth rate is at its maximum during the initial warm up from a cold start. Thus, only the minimum leak down rate is available at the time the maximum leak down rate is required. Current lash adjusters are unable to provide the required leak down rate during the initial warm up from a cold start.
Similarly, a leak down rate that is too fast can cause a hydraulic lash adjuster to relax sufficiently during a single cycle causing the cam follower to lose contact with the cam. Under this circumstance, the engine valve could potentially slam shut, causing noise which is most evident under hot idle conditions. Since the leak down rate varies with engine fluid viscosity, both the grade of engine fluid used and the temperature will affect the leak down rate, with the result that there may not be a single leak down rate setting that is satisfactory under all conditions. For example, as the engine's oil gets hotter and less viscous, the leak down rate increases. However, the engine valve train growth rate is at its minimum during hot running conditions. Thus, the maximum leak down rate is available at the time the minimum leak down rate is required. Therefore, the current lash adjuster mechanisms do not adequately compensate for lash under all engine parameters and conditions.